The CO2 laser (carbon dioxide laser) is a molecular gas laser based on a gas mixture as the gain medium, which contains carbon dioxide (CO2), helium (He), nitrogen (N2), and possibly some hydrogen (H2), oxygen (O2), water vapor and/or xenon (Xe). Such a laser is electrically pumped via an electrical gas discharge, which can be operated with DC current, with AC current (e.g. 20–50 kHz) or in the radio frequency (RF) domain.
Although direct excitation of CO2 molecules into the upper laser level is possible, it has proven to be most efficient to use a resonant energy transfer from nitrogen molecules. Here, nitrogen molecules are excited by the electric discharge into a metastable vibrational level and transfer their excitation energy to the CO2 molecules when colliding with them. The exited CO2 molecules then largely participate in the laser transition. Helium serves both to depopulate the lower laser level and to remove the heat. Other constituents such as hydrogen or water vapor can help (particularly in sealed-tube lasers) to reoxidize carbon monoxide (CO, formed in the discharge) to carbon dioxide.
CO2 lasers typically emit at a wavelength of 10.6 μm, but there are dozens of other laser lines in the region of 9–11 μm (particularly at 9.6 μm). This is because two different vibrational states of the CO2 molecules can be used as the lower level, and for each vibrational state, there is a substantial number of rotational states, leading to many sub-levels. Dipole transitions (the only ones with a relatively high strength) are possible with ΔJ = ±1, where ΔJ = 1 (R branch) leads to higher photon energies (shorter wavelengths) and ΔJ = −1 (P branch) to lower energies:
- Transitions of the stronger band, involving one of the two possible final vibrational levels, have their P branch around 10.6 μm, with P20 being the dominant transition, and the R branch around 10.2 μm.
- Transitions of the other band have the P branch around 9.6 μm and the R branch around 9.3 μm.
With a suitable wavelength tuning element in the laser resonator, a CO2 laser can be made to lase on one of more than a dozen transitions with relatively closely spaced wavelengths in each branch, but continuous wavelength tuning is not possible due to the discrete rotational states of the molecules. Without a wavelength-selective element in the resonator, one may obtain simultaneous lasing on a few transitions, or occasional jumps to other transitions during operation.
Output Powers and Efficiency
In most cases, average output powers are between some tens of watts and many kilowatts. The power conversion efficiency can be around 10% to 20%, i.e., it is higher than for most gas lasers (due to a particularly favorable excitation pathway), also higher than for lamp-pumped solid-state lasers, but lower than for many diode-pumped lasers.
Due to their high output powers and long emission wavelengths, CO2 lasers require high-quality infrared optics, often made of materials like zinc selenide (ZnSe) or zinc sulfide (ZnS).
CO2 Laser Types
The family of CO2 lasers is very diverse:
- For laser powers between a few watts and a several hundred watts, it is common to use sealed-tube or no-flow lasers, where the laser bore and gas supply are contained in a sealed tube. Waste heat is transported to the tube walls by diffusion (with a quite helpful effect of the helium) or a slow gas flow. Such lasers are compact and rugged, and reach operation lifetimes of several thousands of hours. Here, one needs to employ methods for continuously regenerating the gas – in particular, for counteracting the dissociation of CO2 by catalytic re-oxidization of CO. The beam quality can be very high.
- High-power diffusion-cooled slab lasers (not to be confused with solid-state slab lasers) have the gas in a gap between a pair of planar water-cooled RF electrodes. The excess heat is efficiently transferred to the electrodes by diffusion, if the electrode spacing is made small compared with the electrode width. For efficient energy extraction, one often uses an unstable resonator with output coupling at the side of a highly reflecting mirror. Several kilowatts of output are possible in conjunction with reasonable beam quality.
- Fast axial flow lasers and fast transverse flow lasers are also suitable for multi-kilowatt continuous-wave output powers and high beam quality. The excess heat is removed by the fast-flowing gas mixture, which passes an external cooler (heat exchanger) before being used again in the discharge. The gas can be continuously regenerated and occasionally replaced. Transverse flow lasers reach highest output powers, but typically with lower beam quality.
- Transverse excited atmosphere (TEA) lasers have a very high (about atmospheric) gas pressure. As the voltage required for a longitudinal discharge would be too high, transverse excitation is done with a series of electrodes along the tube. TEA lasers are operated in pulsed mode only, as the gas discharge would not be stable at high pressures. They often produce average output powers below 100 W, but can also be made for powers of tens of kilowatts (combined with high pulse repetition rates).
- There are gas dynamic CO2 lasers (a kind of chemical lasers) for multi-megawatt powers (e.g. for anti-missile weapons), where the energy is not provided by a gas discharge but by a chemical reaction in a kind of rocket engine.
These concepts lead to quite different laser architectures, with specific strengths and weaknesses concerning output power potential, beam quality, gas consumption and device lifetime.
Applications of CO2 Lasers
- cutting plastic materials, wood, die boards, etc., exhibiting high absorption at 10.6 μm, and requiring moderate power levels of 20–200 W
- cutting and welding metals such as stainless steel, aluminum or copper, applying multi-kilowatt powers
- laser marking of various materials
Competition with Solid-state Lasers
CO2 lasers used for laser material processing (e.g. welding and cutting of metals, or laser marking) are in competition with solid-state lasers (particularly YAG lasers and fiber lasers) operating in the 1-μm wavelength regime. These shorter wavelengths have the advantages of more efficient absorption in a metallic workpiece and the potential for beam delivery via fiber cables. (There are no optical fibers for high-power 10-μm laser beams.) Besides, 1-μm beams can be more tightly focused, provided that the beam quality is high. However, the latter potential normally cannot be realized with high-power lamp-pumped lasers, and diode-pumped lasers tend to be more expensive. Concerning absorption, CO2 laser beams are actually quite favorable for certain materials like polymers.
For these reasons, CO2 lasers are still widely used in the cutting and welding business, particularly for parts with a thickness greater than a few millimeters, and their sales still make a substantial part of all global laser sales. This may to some extent change in the future due to the development of high-power thin-disk lasers and advanced fiber cables in combination with techniques which exploit the high beam quality of such lasers.
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